The present invention relates to temperature compensation of optical systems and, more particularly, to the use of the thermal characteristics of a laser beam source module to compensate for temperature effects on the entire optical system in which it is used. The laser beam source module comprises a semiconductor laser diode, a collimating lens and the mechanical structure to provide proper spacing and support of the components.
It is well known that the imaging performance of an optical system is dependent on temperature. Increases and decreases in ambient temperature changes the physical dimensions of both the lens elements as well as the mechanical components of the optical system. Also, the refractive properties of the lens elements change with changes in ambient temperature. For systems using semiconductor laser diodes, there is an additional temperature dependency, namely, that the emitted wavelength is a function of the ambient temperature. These wavelength variations affect the imaging performance of systems that depend on the relationship of refractive index to wavelength.
Many prior art techniques have successfully compensated for some or all of the above temperature dependent factors in an optical device to maintain its original imaging performance. Most of these techniques are expensive and incorporate complicated linkages or use of fluids. Representative examples of systems that maintain beam collimation are shown in U.S. Pat. Nos. 4,720,168, 4,855,987 and 4,815,059. U.S. Pat. No. 4,236,790 illustrates an optical apparatus that maintains a fixed mechanical distance between two surfaces such as, a lens and an image plane, so that the distance between them is independent of ambient temperature. The predetermined distance is maintained between the two surfaces by actively heating or cooling the apparatus. The temperature compensation technique of the '790 patent is similar to those described in the '168, '987 and '059 patents only it is reversed in the sense of maintaining a fixed focus and not necessarily collimation.
Temperature compensated devices often are incorporated in larger and more complicated systems. In such systems, the temperature compensated device compensates for itself while the rest of the optical system suffers from all of the temperature dependent factors mentioned above.
It is, accordingly, a general object of the invention to provide an athermalized optical system.
It is another object of the invention to provide an athermalized optical system in which a thermally compensated system component compensates for mechanical and optical thermal effects of the entire optical system.
It is still another object of the invention to provide an athermalized optical system that maintains focus on the last surface of the system taking into account all of the effects of temperature in the system.
It is a further object of the invention to provide an athermalized imaging system having a laser beam generating module, a collimator, a rotating mirror, scan lens and media transport.
It is a feature of the invention that the athermalization of the optical system can be accomplished in an inexpensive manner using a minimum of standard structural materials.
It is another feature of the invention that the athermalization is achieved without "actively" moving or sliding parts, without expensive linkages and without heating or cooling components of the optical system.